Field of the Invention
The present invention relates to a polycrystalline diamond body, a cutting tool, a wear-resistant tool, a grinding tool, and a method for producing a polycrystalline diamond body. More particularly, the invention relates to a polycrystalline diamond body useful as a cutting tool, a wear-resistant tool, and a grinding tool, the cutting tool, the wear-resistant tool, and the grinding tool as well as a method for producing the polycrystalline diamond body.
Description of the Background Art
A sintered diamond body used for conventional diamond tools is obtained using a metal such as cobalt (Co) or the like, and a ceramic such as silicon carbide (SiC) or the like, as a sintering aid and a binder. Further, Japanese Patent Laying-Open No. 4-074766 and Japanese Patent Laying-Open No. 4-114966, for example, disclose using carbonates as sintering aids. According to these documents, a sintered diamond body is obtained by sintering diamond powder along with a sintering aid and a binder under stable high-pressure and high-temperature conditions in which diamond is thermodynamically stable (generally, pressure 5 to 8 GPa, temperature 1300-2200° C.). On the other hand, naturally occurring polycrystalline diamond bodies (carbonado and ballas) are also known, and some of them are used as drill bits. These polycrystalline diamond bodies, however, are not used for industrial purposes very often, since they vary significantly in material quality, and can only be found in limited quantities.
A polycrystalline diamond body obtained using a sintering aid contains the sintering aid used, which may act as a catalyst promoting graphitization of diamond. As a result, the heat resistance of the resulting polycrystalline diamond body deteriorates. Further, when heat is applied to the polycrystalline diamond body, fine cracks tend to develop due to a difference in thermal expansion between the catalyst and the diamond. As a result, the mechanical properties of the polycrystalline diamond body deteriorate.
Polycrystalline diamond bodies are also known from which the metal present at grain boundaries of diamond particles has been removed to improve the heat resistance. Although this method improves the heat-resistant temperature to about 1200° C., the polycrystalline body becomes porous and thus, has further decreased strength. A polycrystalline diamond body obtained using SiC as a binder has high heat resistance, however, it has low strength because diamond particles are not bonded together.
On the other hand, a method is known in which non-diamond carbon such as graphite, amorphous carbon, or the like is directly converted into diamond at an ultra-high pressure and a high pressure, without using a catalyst and/or a solvent, and sintered simultaneously (direct conversion and sintering method). J. Chem. Phys., 38 (1963) pp 631-643, Japan. J. Appl. Phys., 11 (1972) pp. 578-590, and Nature 259 (1976) p. 38, for example, have shown that a polycrystalline diamond body is obtained using graphite as a starting material under an ultra-high pressure of 14 to 18 GPa and a high temperature of 3000 K and more.
However, in the production of a polycrystalline diamond body according to all of J. Chem Phys., 38 (1963) pp. 631-643, Japan J. Appl. Phys., 11 (1972) pp. 578-590, and Nature 259 (1976) p. 38, a method of heating by direct current passage is used in which conductive non-diamond carbon such as graphite or the like is heated by directly passing current therethrough. The polycrystalline diamond body thus obtained contains remaining non-diamond carbon such as graphite or the like, and also has a nonuniform crystal grain size of diamond. As a result, the polycrystalline diamond body has poor hardness and strength.
Thus, in order to improve the hardness and strength, New Diamond and Frontier Carbon Technology, 14 (2004) p. 313 and SEI Technical Review 165 (2004) p. 68 have shown a method for obtaining a dense and high-purity polycrystalline diamond body by a direct conversion and sintering method, in which high-purity graphite as a raw material is indirectly heated at an ultra-high pressure of 12 GPa or more and a high temperature of 2200° C. or more.